1 Introduction

The proliferation of electronic devices and wireless technology, utilized for both civilian and military purposes, has led to a significantly increased presence of potentially harmful electromagnetic waves (EMW) in our society compared to previous years. Prolonged exposure to EMW may negatively impact our health by elevating heart rates and compromising the body’s immune system [1,2,3,4]. Addressing this concern necessitates the development of a novel electromagnetic microwave absorber with specific attributes, including optimal microwave absorption capabilities, broad absorption across different frequencies, lightweight design, resilience to corrosion, and thermal stability [5,6,7,8].

According to the aforementioned data about the best requirements that need to have a desirable microwave-absorbing material, carbon-based materials have been candidates because they have adjustable features, a very low density, chemical stability, an easy way of manufacturing, and also a low cost [9,10,11,12]. However, carbon-based materials demonstrate dielectric loss behavior in addition to high complex permittivity, which leads to a mismatch in impedance with the incident EMW, and as a consequence, reflection of the EMW occurs at the surface of the shielding material [13,14,15].

Consequently, two distinct approaches pave the way for optimizing the electromagnetic wave (EMW) attenuation capabilities of carbon-based materials, thereby enhancing absorption efficiency. The initial approach entails incorporating a magnetic element to enhance the composite permeability. This augmentation aims to improve the magnetic loss characteristic, fostering a more favorable balance with the inherent dielectric loss characteristic within the material [16,17,18]. The second pathway entails implementing an intelligent design principle in constructing the material's structure, emphasizing specific features such as porous structures. This approach holds promise as it offers a systematic method to craft a microwave-absorbing material with the sought-after shielding effectiveness [19, 20].

Metal–organic frameworks (MOFs) represent a category of porous materials comprised of metal ions or metal oxide clusters connected by organic ligands. Due to their low weight, expansive specific surface area, and diverse topological structures, these materials have garnered extensive usage in applications such as gas sensors, photocatalysis, drug delivery carriers, and supercapacitors [21,22,23]. In the fields of electromagnetic interference (EMI) shielding and microwave absorption, the use of MOF-derived materials is now considered a promising applicable technique [24, 25]. The approach involves utilizing a metal–organic framework (MOF) as the source for a carbon structure backbone, incorporating magnetic metal oxide through a pyrolysis process conducted at specific temperatures for a defined duration. Subsequently, the synthesized composites with pyrolytic MOF exhibit well-dispersed magnetic metallic components within porous carbon. This heterogenous and well-ordered carbon structure, which remains after the pyrolysis process, coupled with the integration of magnetic clusters, has the potential to significantly enhance electromagnetic interference (EMI) shielding effectiveness [26,27,28,29]. The synergistic optimization of magnetic loss and dielectric loss properties is anticipated to enhance the overall shielding performance of the proposed material [17, 30, 31]. Achieving an optimal balance between magnetic loss and dielectric loss remains a significant challenge in obtaining materials with high electromagnetic interference (EMI) shielding. Despite this challenge, recent literature has shown a notable focus on investigating metal–organic frameworks and graphene composite materials as promising candidates for effective microwave absorption materials [32,33,34,35,36,37,38]. They mainly suggest a microwave absorption material based on reflection loss (RL) alone, without taking into consideration the transmission loss factor (TL). According to the precise handling of EMI shielding data, it is obvious that the accurate expression of the total EMI shielding is mainly the net results of both reflection (based on RL) and transmission (based on TL) [39,40,41,42,43,44]. The multiple internal reflections (M) can be neglected when the transmission loss is greater than 10 dB; hence, the reflection and transmission losses are the principal and essential parameters to evaluate the shielding performance processes [45]. Also, it becomes insignificant when the SEA is more than 6 dB at higher frequencies (i.e., above 20 kHz); therefore, it will not be considered in the current research considering the microwave X-band [46,47,48,49]. It is worth noting that absorption losses rely on the physical properties of the shielding material. The phenomenon of absorption loss arises due to the generation of induced currents inside the medium, resulting in ohmic losses and also subsequent heating of the shielding material [50]. Conversely, the reflection loss value of the shielding material is influenced by the relative mismatch between incident electromagnetic waves (EMW) and the impedance of the shielding material, irrespective of the EMW source [51]. Additionally, papers should distinctly illustrate the presence of reduced graphene oxide (RGO) in the composite structure after implementing the calcination process. This is crucial for establishing the effectiveness of the resulting material for electromagnetic interference (EMI) shielding [52,53,54]. The temperature treatment is detrimental to its structure and quality [17, 55]. Above 500°C, even under inert ambient conditions, the graphene starts to deteriorate due to the decomposition and sublimation of the carbon backbone [56,57,58].

This can be proved by delivering high-resolution SEM and TEM images and RAMAN spectroscopy. The RAMAN spectrometer serves as a potent instrument for discerning the bonds and structural characteristics of materials associated with graphite and graphene. It does so by detecting the distinct peaks referred to as the D and G bands, positioned at 1360 and 1600 Raman shifts (cm−1), respectively. Conducting Raman analysis is an indispensable step in identifying the presence of Reduced Graphene Oxide (RGO) within the composite. This analysis offers valuable insights into various aspects of RGO and detecting the sp2 hybridized carbon–carbon bond in the graphitic structure [59,60,61,62].

This study aims to explore a novel method for preparing a magnetic metal–organic framework (MOF), specifically MIL-53(Fe), in combination with reduced graphene oxide (RGO). The process involves creating a MIL-53(Fe)/graphene oxide (GO) composite as an intermediate step under hydrothermal conditions with continuous mixing, ensuring homogeneity at varying GO loading percentages. Subsequently, a pyrolysis process at 450 °C for an hour is employed in a tubular furnace under ambient inert gas (Ar) to exfoliate the GO into reduced graphene oxide (RGO), minimizing thermal effects that could compromise the RGO structure. The resulting samples, namely P-MIL-53(Fe)/RGO15, P-MIL-53(Fe)/RGO20, and P-MIL-53(Fe)/RGO30, undergo characterization and evaluation as potential candidates for electromagnetic interference (EMI) shielding. This study considers both reflection and transmission losses, interpreting the shielding effectiveness values of the examined composites to assess their absorption behavior. The findings related to electromagnetic wave (EMW) attenuation suggest that the tailored synthesis protocol holds significant promise for producing an efficient MOF/RGO composite with a high production yield suitable for large-scale industrial manufacturing. This composite material exhibits broad applicability in both civilian and military domains, including EMI absorbers, paints, and adhesives.

2 Experimental work

The following chemicals have been used (purchased from Alfa Aesar) for the synthesis of graphene oxide (GO) and MIL-53(Fe): graphite powder (99.8%), H2SO4 (98%), H3PO4 (85%), KMnO4 (99%), H2O2 (10% solution), FeCl3⋅6H2O (98%), 1,4-benzene dicarboxylic acid (H2BDC, 98%), and N, N-dimethylformamide (DMF, 99.8%).

2.1 Synthesis of GO and MIL-53(Fe)

Graphene oxide (GO) was synthesized using the improved Hummer method [21]. Graphite powder (5 g) was combined with H2SO4 (593 ml) and H3PO4 (74 ml) in a beaker (500 ml). To prevent particle aggregation and heat accumulation and to ensure a complete dispersion, KMnO4 (30 g) was added gradually while agitating to ensure a uniform mixture. After cooling, the mixture was stirred for another three days until the thick GO product formed. Then, the mixture was added gradually to cold distilled water (at 5 °C) while stirring with a mechanical stirrer at 400 rpm for an hour. Following that, H2O2 (10% solution) was added dropwise until the solution color changed to the known yellow color of GO. To remove any precursor remains, the GO particles were washed and decanted three times with HCl (15%) before being centrifuged and dried at 45 °C for 48 h [44, 63].

The production process of MIL-53(Fe) was carried out via the solvothermal technique approach [64]. FeCl3⋅6H2O (13.5 g) and H2BDC (8.2 g) were combined in a 1-l Pyrex bottle containing DMF (250 ml) and a magnetic bar. The mixture solution was stirred for 30 min before putting the setup in an oil bath, as shown in Fig. 1. The mixture was heated at 120 °C for 15 h under continuous mixing, producing MIL-53(Fe) fine powder suspended in reaction medium. The setup depicted in Fig. 1 offers several advantages, including ease of operation, safety, precise control over experimental temperature, continuous mixing to yield a homogeneous mixture, uniform heat transfer to all constituents, and scalability for industrial applications. This stands in contrast to traditional synthesis techniques found in the literature, which rely on autoclave devices and are limited in their ability to produce only a few quantities of the product.

Fig. 1
figure 1

MIL-53(Fe) and MIL-53(Fe)/GO synthesis setup

Full size image

Finally, the synthesized MIL-53(Fe) powder was filtered and collected using a Buchner funnel and then washed with methanol and acetone. MIL-53(Fe) was dried inside an oven at 45 °C for a 4-h. The obtained product, weighing 8 g with a theoretical yield of 11 g and a percent yield of 72.7%, surpasses the quantities reported in the literature, particularly when employing the autoclave technique in metal–organic framework (MOF) synthesis. This notable difference underscores the effectiveness of the described method, which not only yields a higher quantity but also offers advantages in terms of simplicity, safety, and scalability for industrial applications [65, 66].

2.2 Synthesis of MIL-53(Fe)/GO and P-MIL-53(Fe)/RGO

To produce a MIL-53(Fe)/GO composite, a solution was prepared by dissolving (13.5 g) of FeCl3⋅6H2O and (8.2 g) of H2BDC in 250 ml of DMF. The resulting mixture was stirred for one hour at ambient temperature. Subsequently, a predetermined amount of GO was introduced into the solution, and a 20-min ultrasonic blending process was employed to homogenize the solution.

Subsequently, the mixture was transferred into a 1-l Pyrex bottle and agitated for 30 min, as illustrated in Fig. 1. The experimental setup was then immersed in an oil bath and subjected to continuous heating at a stable temperature of 120 °C for 15 h under continuous mixing to achieve the desired homogeneity of the composite. Additionally, the use of the traditional hydrothermal approach (autoclave device method) for preparing MIL-53(Fe)/RGO over an extended period without mixing raises concerns about the homogeneity of the final product.

After cooling to ambient temperature, the resulting powdered sample was obtained by filtration using a Buchner funnel. The sample was then subjected to purification with methanol and acetone. Following purification, the specimen underwent a drying process in an oven set at 45 °C. The mass ratios of GO to MIL-53(Fe) were 15%, 20%, and 30%, respectively labeled as MIL-53(Fe)/GO15, MIL-53(Fe)/GO20, and MIL-53(Fe)/GO30 [67].

All the previously synthetic composites (MIL-53(Fe)/GO15, MIL-53(Fe)/GO20, and MIL-53(Fe)/GO30) were subjected to the pyrolysis process in a tubular furnace at 450 °C for an hour, under the flow of inert gas (Ar), to produce P-MIL-53(Fe)/RGO15, P-MIL-53(Fe)/RGO20, and P-MIL-53(Fe)/RGO30, respectively. These conditions were applied to maintain, as much as possible, the MOF and RGO's proper structure and avoid backbone sublimation [68,69,70,71,72].

2.3 Electromagnetic interference shielding calculations

The correct assessment of an absorbing material can be straightforwardly conducted by employing the S parameters (e.g., S11, S12, S21, and S22) derived from VNA (Vector Network Analyzer) outcomes through the utilization of the subsequent equations:

Equations 1 through 6 articulate the correlation between the S parameters and the reflection, absorption, and overall shielding efficiencies of the investigated material [73, 74].

$$T= {\left|\frac{{E}_{T}}{{E}_{i}}\right|}^{2}={\left|{S}_{12}\right|}^{2}= {\left|{S}_{21}\right|}^{2}$$
(1)
$$R= {\left|\frac{{E}_{R}}{{E}_{i}}\right|}^{2}={\left|{S}_{11}\right|}^{2}= {\left|{S}_{22}\right|}^{2}$$
(2)
$$A= 1-R-T$$
(3)
$${SE}_{R}=-10\,{\text{log}}\left(1-R\right)$$
(4)
$${SE}_{A}=-10\,{\text{log}}\left(\frac{T}{1-R}\right)$$
(5)
$${SE}_{T}= {SE}_{A}+{SE}_{R}+{SE}_{M}$$
(6)

Here A, R, and T denote the coefficients for absorbance, reflectance, and transmittance, respectively, while S represents the scattering parameters. SET, SER, and SEA signify the total, reflection, and absorbing shielding efficiencies.

When total shielding effectiveness (SET) exceeds 10 dB, the impact of multiple reflection loss (SEM) can be deemed negligible [75]. Electromagnetic interference (EMI) shielding materials achieving a SET surpassing 50 dB equate to an effective shielding of 99.9% against incident electromagnetic waves, attributed to a synergistic blend of absorption and reflection capabilities [76].

3 Characterization

An X-ray powder diffractometer (Panalytical X'PERT PRO MPD, England) was operated to perform powder X-ray diffraction analysis (PXRD) to collect the spectra of synthetic materials. During the experimental trials, a radiation source emitting CuKα1 radiation with a wavelength of 1.5406 Å was used at a voltage of 40 kV and a current of 40 mA. The duration of each scan was 0.4 s, including a temperature range of 4–80 °C and achieving a precision of 0.02 degrees. A dispersive Raman microscope instrument (with model Sentera II, Bruker optics, Germany) with a resolution of 4 cm−1 was utilized to generate Raman spectra for synthesized composites. A Nikon 20 objective lens and a Neodymium-doped Yttrium Aluminum Garnet (Nd: YAG) excitation source with a wavelength of 532 nm and a power of 10 mW were used to focus the laser beam. A Fourier transform infrared (FTIR) study was performed in the 400–4000 cm−1 region at a resolution of 4 cm−1 using a JASKO 4100 spectrometer (Japan). By integrating 100 images, accurate spectra (having a good SNR) were obtained for every sample. The samples’ morphology was studied using a scanning electron microscope (German-type Zeiss EVO-10). A thermogravimetric analysis determined the synthesized samples’ thermal stability (model TGA-60, Shimadzu, Japan). The TGA was conducted at an ambient temperature of 600 °C at a heating rate of 10 °C/min in an inert atmosphere. A vector network analyzer (HP 8510 C) was utilized with 2 ports to evaluate the shielding capability of the synthesized composites over the 8 to 12 GHz frequency range through measuring the S parameters. Tests were conducted using a standard rectangular waveguide flange (WR-90) with dimensions of 22.86 mm and 10.16 mm in accordance with the used waveguide, which was compressed to 10 tons. The electrical conductivities of the samples were measured according to ASTM D257. A voltage in the range of − 0.1 to 0.1 V was provided to the four-probe electrode model Keithley 220 using a power source (Zahner Mess Technic, Model IM6ex potentiostat, Gundelsdorf, Germany) and the specimen under testing. For every specimen, five measurements were recorded, and the average conductivity value was calculated.

4 Results and discussion

X-ray diffraction (XRD) analysis was employed to validate the successful preservation of the MIL-53(Fe) backbone structure after pyrolysis. The peaks of the synthesized (syn) MIL-53(Fe), GO, and MIL-53(Fe)/GO (15, 20, and 30) composites are illustrated in Fig. 2A. The pattern of the synthesized MIL-53(Fe) aligns with those reported in the literature, confirming the successful preparation of the material [77, 78]. In accordance with Bragg's rule, GO exhibits a single peak at (2θ = 10.2°), which equates to an interlayer distance of 8.8 Å. Intercalation of oxygen functional groups, including hydroxyl, epoxy, and carbonyl, during the synthesis process is responsible for the greater distance between adjacent layers in the graphene oxide structure compared to graphite (3.3 Å) [79]. The principal diffraction peak at (2θ = 9.2°) in composites, MIL-53(Fe)/GO (15, 20, and 30), was divided into two peaks, and a new peak appeared at (2θ = 9.6°), which is correlated to the deformation of the MIL-53(Fe) crystalline lattice. In addition to that higher intensity, the peak at (2θ = 12.5°) for MIL-53(Fe)/GO20, while a lower intensity was observed in the peak at (2θ = 9.2°) [65, 80]. Modifications were also found in the X-ray diffraction patterns for several MOF/GO composites, including MOF-5/GO [81], MIL-100(Fe)/GO [82], and MOF-benzoic acid functionalized graphene [83]. In these composites, the incremental addition of GO resulted in the disappearance of certain peaks and the appearance of others. This observation led to the belief that GO played a role in modifying the crystalline structure of MOFs [65].

Fig. 2
figure 2

XRD patterns of A Before pyrolysis process B After pyrolysis process. (Inset: Sim MIL-53(Fe) reproduced via Mercury/CSD file: 797263)

Full size image

Figure 2B Demonstrate the X-ray diffraction peaks of synthesized MIL-53(Fe), P-MIL-53(Fe), GO, and P-MIL-53(Fe)/RGO (15, 20, and 30). The peaks at 2θ = 30.1, 35.5, 44, 53.5, 57.2, and 63.6 relate to the crystallographic planes of γ-Fe2O3 at (220), (311), (400), (422), (511), and (440) respectively (JCPDS 25-1402) [84]. At 450 °C (i.e. after the pyrolysis process), GO undergoes reduction and transforms into RGO, which lacks a distinctive peak in XRD spectra. However, the reduced graphene oxide diffraction peaks are difficult to discern in all samples. The observed phenomenon might perhaps be accounted for by the decreased diffraction intensity or the absence of a stacked structure in all samples of reduced graphene oxide (RGO) [85]. According to the formerly mentioned data, there is no visible peak for RGO in the composites [86, 87].

Major peaks of P-MIL-53(Fe) and P-MIL-53(Fe)/RGO (15, 20, and 30) correspond with peaks of simulated γ-Fe2O3 and simulated (sim) MIL-53(Fe), showing the lack of impurities in the synthesized samples. Nevertheless, the main peak at (2θ = 9.2°) of synthesized MIL-53(Fe) shifted to (2θ = 8.3°) in the case of P-MIL-53(Fe)/RGO (15, 20, and 30), indicating that the volume expansion that occurs in the lattice structure was caused by the creation of inner strain at higher temperatures throughout the composite fabrication [71]. The XRD results affirm the successful preservation of the MIL-53(Fe) backbone structure and the incorporation of γ-Fe2O3 after pyrolysis, marking an achievement that sets this work apart from numerous previous studies. Many of those studies struggled to uphold structural heterogeneity and resulted in carbon derived solely from MOF [88,89,90,91].

Figure 3A–C shows the morphological characterization through TEM for MIL-53(Fe)/GO (15, 20, and 30), revealing a structured arrangement where MIL-53(Fe) particles are linked with GO sheets, forming a distinctive sandwich-like configuration. The thin layers of GO act as efficient dividers, underscoring the organized nature of the MIL-53(Fe) and GO components. Unlike a random combination, the images illustrate a deliberate and structured assembly. With an increasing percentage of GO from 15 to 30%, the dispersion of MIL-53(Fe) particles becomes less uniform, and a higher concentration of GO disrupts the crystalline structure. This effect is particularly evident in MIL-53(Fe)/GO30, where MIL-53(Fe) particles are partially obscured by GO layers [92].

Fig. 3
figure 3

TEM images of A MIL-53(Fe)/GO15, B MIL-53(Fe)/GO20, C MIL-53(Fe)/GO30, D P-MIL-53(Fe)/RGO15, E P-MIL-53(Fe)/RGO20, and F P-MIL-53(Fe)/RGO30

Full size image

Figure 3D–F presents the detailed morphology and structure of P-MIL-53(Fe)/RGO (15, 20, and 30). A uniform distribution of particles is observed on the RGO surface, formed after the pyrolysis process at 450 °C for the synthesized composites, resulting in the conversion of iron ions (Fe+2) in the nodes of MIL-53(Fe) to γ-Fe2O3. Notably, the structural integrity of RGO remains intact post-pyrolysis, contributing to enhanced electrical conductivity of the synthesized composites. This preservation of RGO structure is advantageous for improving the shielding effectiveness, rendering the materials suitable for applications in microwave absorption [93, 94].

Raman spectra for GO, MIL-53(Fe), and MIL-53(Fe)/GO (15, 20, and 30) composites are shown in Fig. 4A Graphene oxide contains two prominent peaks in its spectrum, the G and D bands. A defect in the graphitic structure was identified by the existence of the D band, and the existing graphite was indicated by the presence of the G band due to the in-plane vibration of the C–C bond caused by the sp2 orbital. Both the D band at 1340 cm−1 and the G band at 1575 cm−1 were consistently observed features in the GO spectra [79, 95]. In the 400–2000 cm−1 range, the vibration modes related to the organic component H2BDC of MOF material dominate the spectra of MIL-53(Fe). The Raman spectra of the three composites are identical, and the major peaks of MIL-53(Fe) are preserved, yet there remain some distinctions between composites and MIL-53(Fe). The appearance of the D band of GO is evident at 1350 cm−1. However, the absorption band at 1575 cm−1 is not observed, as the G band of GO overlaps with the MIL-53(Fe) band. These characteristics demonstrate the presence of both GO and MIL-53(Fe) units in the composites [65, 96].

Fig. 4
figure 4

Raman spectra of A GO and MIL-53(Fe)/GO (15, 20, and 30), and B GO and P-MIL53(Fe)/RGO (15, 20, and 30)

Full size image

In Fig. 4B, the Raman spectra of P-MIL-53(Fe)/RGO (15, 20, & 30) reveal two prominent bands at 1350 cm−1 and 1590 cm−1, corresponding to the D and G bands, respectively. The ratio of the intensities of these bands (ID/IG) is indicative of the structural defects and disordered states in carbon substances.

The ID/IG values of P-MIL-53(Fe)/RGO (15, 20, and 30) composites exhibit a decrease as the RGO ratio increases from 15 to 20%, as observed in P-MIL-53(Fe)/RGO15 and P-MIL-53(Fe)/RGO20. However, these values start to rise again with the addition of RGO at a weight ratio of 30% in P-MIL-53(Fe)/RGO30. This suggests that an optimal RGO quantity is achieved at a 20% weight ratio, where disorder decreases. Conversely, increasing the RGO ratio to 20% for P-MIL-53(Fe)/RGO20 may contribute to a reduction in structural defects and an enhancement in the graphitization degree of the composites [97, 98].

The ID/IG intensity ratio provides valuable insights into the reduction strategy, effectively eliminating oxygen functional groups responsible for introducing defects in the carbon lattice of graphite. Due to the strong oxidation employed in the improved Hummer method, the D band in graphene oxide (GO) Raman spectra significantly expands, resulting in an ID/IG ratio of 0.8. Following the reduction process, the ID/IG ratios for P-MIL-53(Fe)/RGO15, P-MIL-53(Fe)/RGO20, and P-MIL-53(Fe)/RGO30 were 0.79, 0.55, and 0.67, respectively. These values indicate the reduction of oxygen functional groups located on the surface of graphene oxide, leading to the formation of reduced graphene oxide. [99].

The FTIR spectra of MIL-53(Fe) and MIL-53(Fe)/GO (15, 20, and 30) composites are presented in Fig. 5A. The graphene oxide spectrum reveals absorption peaks corresponding to stretching vibrations for O–H at 3200 cm−1, C=O at 1720 cm−1, and C=C bond at 1609 cm−1. Peaks at 1169 cm−1 and 1030 cm−1 are attributed to the stretching vibration modes of epoxy and alkoxy groups (C–O–C and C–O), respectively. The presence of these functional groups confirms the successful synthesis of GO [92, 100].

Fig. 5
figure 5

FTIR OF A MIL-53(Fe)/GO (15, 20, and 30) & GO, B P-MIL-53(Fe) and P-MIL-53(Fe)/RGO (15, 20, and 30)

Full size image

In the FTIR spectrum of MIL-53(Fe), the presence of a short and weak band at 532 cm−1 indicates the vibration of Fe–O bonds. The peak at 747 cm−1 corresponds to the C–H bonding vibration of benzene rings. A relatively intense absorption band at 888 cm−1 signifies the occurrence of C–N vibrations. Additionally, the detected peak at 1012 cm−1 is attributed to the presence of the benzene ring group in the structure.

The existence of the dicarboxylate linker is confirmed by the bands seen at 1383 cm−1 and 1589 cm−1, which are attributed to asymmetric (C–O) and symmetric (C–O) vibrations, respectively.

The composite materials of MIL-53(Fe)/GO (15, 20, and 30) exhibit a peak at around 1532 cm−1, indicating a blue shift in the (O–C=O) bond. This shift suggests the successful growth of MIL-53(Fe) on the surface of graphene oxide. Characteristic MIL-53(Fe) peaks were mostly seen in MIL-53(Fe)/GO (15, 20, and 30) composites, showing that the MIL-53(Fe) crystalline structure was preserved in the composite[67, 100].

The FTIR spectra of the P-MIL-53(Fe) and P-MIL-53(Fe)/RGO (15, 20, and 30) composites are shown in Fig. 5B. All samples have distinct peaks at 1546 and 1365 cm−1, both of them indicate the existence of a dicarboxylate linker. Additionally, the vibration mode of Ar–C–H is accountable for the distinguished bands at 740 cm−1. It seems that the framework of the structure still exists even after the pyrolysis process, which emphasizes that the organic linker is not completely broken down [71].

Figure 6 illustrates the thermogravimetric analysis (TGA) of the studied samples. In Fig. 6A, the TGA results of GO, MIL-53(Fe), and MIL-53(Fe)/GO (15, 20, and 30) composites are presented. The curves obtained exhibit a high degree of thermal stability. In the MIL-53(Fe) sample, the TGA curve indicates some weight loss before 350 °C, likely due to the evaporation of residual solvent. Between 400 and 600 °C, the decomposition of organic ligand species occurs, leading to the collapse of the MIL-53(Fe) structure and a significant decrease in weight. For the GO sample, the initial significant weight loss occurs between 150 and 200 °C, corresponding to the loss of functional groups [101].

Fig. 6
figure 6

TGA of A MIL-53(Fe), GO, and P-MIL-53(Fe)/RGO (15, 20, and 30), B P-MIL-53(Fe) and P-MIL-53(Fe)/RGO (15, 20, and 30)

Full size image

The MIL-53(Fe)/GO (15, 20, and 30) composites exhibited slightly greater weight loss at approximately the same temperature, indicating a strong linkage between GO sheets and MIL-53(Fe). However, at 200 °C, MIL-53(Fe)/GO (15, 20, and 30) composites underwent rapid decomposition, attributed to the elimination of an excess quantity of GO that cannot be firmly linked. The TGA results suggest that MIL-53(Fe)/GO (15 and 20) composites with lower GO concentrations were more thermally stable than MIL-53(Fe)/GO30, making them suitable for industrial applications [101, 102].

In Fig. 6B, P-MIL-53(Fe)/RGO20 exhibits higher thermal stability than P-MIL-53(Fe) and P-MIL-53(Fe)/GO (15, 20, and 30). This is attributed to the possibility that P-MIL-53(Fe)/RGO20 contains the optimum amount of graphene, which has high thermal conductivity, in the composite. This characteristic facilitates regular heat transfer throughout all parts of the composite, contributing to its superior thermal stability compared to other synthesized samples.

4.1 EMI shielding effectivity evaluation

Figure 7 illustrates the shielding effectiveness (SE) for the P-MIL-53(Fe)/RGO (15, 20, and 30) samples with various loading amounts in sample holders (0.5, 1, 1.5, and 2 g) across the frequency range of 8–12 GHz. Additionally, it depicts the electrical conductivity of the P-MIL-53(Fe)/RGO (15, 20, and 30) samples with a 2g loading amount in sample holders.

Fig. 7
figure 7

Shielding effectiveness (SE) of A P-MIL-53(Fe)/RGO15, B P-MIL-53(Fe)/RGO20, C P-MIL-53(Fe)/RGO30, D Electrical conductivity results of P-MIL-53(Fe)/RGO (15, 20, and 30) with 2 g loading amount

Full size image

Figure 7A demonstrates the shielding effectiveness (SE) for the P-MIL-53(Fe)/RGO15 sample with various loading amounts in the sample holder (0.5, 1, 1.5, and 2 g) across the frequency range of 8–12 GHz. The shielding effectiveness (SE) increases with the loading amount in the sample holder. The SE is 15.5 dB at a low loading amount (0.5 g), and it rises to 18 dB, 21.3 dB, and 30.9 dB at 1, 1.5, and 2 g, respectively, corresponding to thicknesses of 2.3, 3.5, and 5 mm. This phenomenon is attributed to the increase in shielding material thickness, leading to an exponential reduction in the strength of electromagnetic waves [103].

The results of the SEA measurements indicate an increase with the rising amount of loading (0.5, 1, 1.5, and 2 g). The SEA is 12.7 dB at a low loading amount (0.5 g), and it increases to 14.2, 17.7, and 27.5 dB at 1, 1.5, and 2 g. On the other hand, SER has a limited impact, increasing by a small amount and reaching (2.8, 1.5, 2, 3.2, and 3.3 dB) with an increase in mass loading to (0.5, 1, 1.5, and 2 g), respectively.

In Fig. 7B, the shielding effectiveness (SE) is depicted across the 8–12 GHz frequency range for the P-MIL-53(Fe)/GO20 sample with various loading amounts in the sample holder (0.5, 1, 1.5, and 2 g). The total shielding effectiveness (SET) increases with a greater amount of loading in the sample holder (0.5, 1, 1.5, and 2 g). Specifically, the SET is 19 dB at a loading amount of 0.5 g, 21 dB at 1 g, 29.5 dB at 1.5 g, 34 dB, and 42.9 dB at 2 g [38, 104].

The measured SEA reveals that as the amount of load in the sample holder (0.5, 1, 1.5, and 2 g) increases, the SEA also shows an increase. Specifically, the SEA is 17.1 dB at a low loading amount (0.5 g), and it increases to 18.9 dB, 27.2 dB, and 40.3 dB at 1, 1.5, and 2 g, respectively.

Figure 7C shows the P-MIL-53(Fe)/RGO30 sample shielding performance behavior with various loading amounts (0.5, 1, 1.5, and 2 g) where the total shielding effectiveness (SET) increases with the loading weight. Specifically, the SET is 21 dB at a low loading amount 0.5 g, rising to 29, 34, and 46.5 dB when the loading amount is increased to 1, 1.5, and 2 g, respectively. This is attributed to the exponential reduction in the strength of electromagnetic waves passing through a medium as the shielding material thickness increases [105, 106]. The SET of P-MIL-53(Fe)/RGO30, measuring 46.5 dB, is observed to be higher than the P-MIL-53(Fe)/RGO (15 and 20), which are measured at 30.9 dB and 42.9 dB, respectively, with a 2g loading amount. This difference is attributed to the increase in the loading amount of RGO from 15 to 30%. As the loading amount increases, the electrical conductivity also increases. Consequently, the interaction with the electric field in the electromagnetic wave intensifies, leading to increased attenuation, as observed in Fig. 7D [17, 41, 107].

Similarly, the results of the SEA measurements rise with the loading amounts. The SEA is 19.3 dB at a low mounting load (0.5 g), and it increases to 27.2, 32.7, and 40.3 dB at 1, 1.5, and 2 g. The absorption mechanism dominates the entire shielding and rises from 80 to 90% due to the absorption effectiveness, whereas SER has a limited impact, increasing by a small amount with increasing mass loading to (1.9, 2.2, 2.8, and 3.1 dB) when increasing the mounting load to (0.5, 1, 1.5, and 2 g), respectively.

The P-MIL-53(Fe)/RGO (15, 20, and 30) composites can be applied as fillers in practical matrices, or potentially as exclusive materials if appropriate production conditions are achieved, to either maintain or enhance the shielding effectiveness (SE) during the manufacturing of electromagnetic (EM) shielding products. Therefore, it is crucial to establish a relationship between the shielding performance and the quantity used per unit surface area (mass/area), commonly known as areal concentration [39, 44].

Figure 8. compiles the average values for SET, SEA, and SER of all P-MIL-53(Fe)/RGO (15, 20, and 30) composites and loadings as a function of areal concentrations, illustrating a generally linear shielding response. With the increase in load in the sample holder, the P-MIL-53(Fe)/RGO (15, 20, and 30) samples exhibited an anticipated enhancement in total shielding efficiency (SET).

Fig. 8
figure 8

Shielding effectiveness (SE) averaged over the tested X-band range of 8–12 GHz: A (SET), B (SER) and C (SEA)

Full size image

In Fig. 8A, the mean shielding effectiveness (SET) for the P-MIL-53(Fe)/RGO30 sample nearly doubled, escalating from 21 dB to 46.5 dB with the increment in sample loading from 0.5 to 2 g. This notable enhancement can be attributed to the augmented thickness of the shielding material, resulting in a pronounced reduction in electromagnetic wave strength while maintaining a consistent cross-sectional area [106, 108].

The results demonstrate that the P-MIL-53(Fe)/RGO30 sample with a loading ratio of 2 g in the sample holder was the most effective in blocking X-band frequencies. The average SET of the P-MIL-53(Fe)/RGO15 is 30.9 dB at a low RGO concentration (15 wt.%), increasing to 42.9 dB and 46.5 dB at P-MIL-53(Fe)/RGO20 and P-MIL-53(Fe)/RGO30, respectively. The EM energy is expected to encounter more reduced graphene oxide particles at a higher content level. Consequently, there will be increased levels of shielding effectiveness because EM waves will undergo more degrees of absorption and reflection [109,110,111].

Figure 8B shows that the average SEA increases from 17.12 dB to 40.5 dB for P-MIL-53(Fe)/RGO20 and from 19.25 dB to 43.5 dB for P-MIL-53(Fe)/RGO30 samples, with increasing mass loading from 0.5 g to 2 g, respectively. This indicates that the absorption mechanism dominates total shielding while Fig. 8C indicates that average SER that rises from 2.84 to 3.35 dB, 1.9 to 3.12 dB, and 1.9 dB to 2.9 dB for P-MIL-53(Fe)/RGO15, P-MIL-53(Fe)/RGO20 and P-MIL-53(Fe)/RGO30 samples respectively according to the loading amounts from 0.5 g to 2 g.

Table 1 illustrates various carbon-based nanocomposite systems reported in the literature.

Table 1 EMI shielding performance based on carbon-based composites
Full size table

The preceding findings suggest that as the quantity of load in the sample holder rises, there is a notable rise in SEA values, emphasizing the increasing importance of the absorption mechanism in overall shielding efficiency. This phenomenon becomes especially apparent as SEA values elevate with the increasing load in the sample holder, highlighting the substantial impact of absorption on EMI shielding.

In contrast, SER values demonstrate a more limited influence, with only slight increments observed. This suggests that while the reflection mechanism contributes to overall shielding, its effect is comparatively subdued when compared to the robust influence of absorption. Our findings emphasize the pivotal role of the absorption mechanism, as represented by SEA values, in in enhancing the EMI shielding effectiveness of the studied material.

5 Conclusion

MIL-53(Fe) and MIL-53(Fe)/GO (15,20, and 30) composites were synthesized using a novel setup which provides a high yield in a single preparation process than the processes reported in the literature. The innovative and unsophisticated approach to produce P-MIL-53(Fe)/RGO was applied while sustaining the composite structural integrity. This setup could allow scaling up the preparation process for the manufacturing field.

Also, we reveal the critical relation of RGO and the thermal treatment during the preparation procedures to maintain the RGO structure with minimal possible defects and maintaining the MIL-53(Fe) framework using our tailored technique by controlling the pyrolysis conditions of the synthesized composites in a tubular furnace to produce the P-MIL-53(Fe)/RGO with different RGO loading percentages (i.e., 15%, 20%, and 30% mass ratios).The synergistic effect between the MOF and RGO enhances the absorption performance and hence the total shielding effectiveness of the developed composite.

The pyrolysis process converts the GO into RGO with minimal possible defects which is essential in the structure of composites and strongly improves the total shielding efficiency. MIL-53(Fe)/RGO30 composite with a loading ratio of 2 g (5 mm thick) has an outstanding total shielding efficiency (SET) of 46.5 dB surpassing other studied composites because the EM energy is predicted to encounter more reduced graphene oxide particles. Consequently, there will be increased levels of shielding effectiveness (SE) because EM waves will undergo more degrees of absorption and reflection. The P-MIL-53(Fe)/RGO is a promising EMI agent using the developed protocol, and its shielding parameters should be studied accurately to optimize the MOF/RGO ratios.